Despite their various differences, all golf clubs share certain fundamental features: they all have a clubhead, a shaft, and a grip. The clubhead is configured for hitting the golf ball. The shaft is a pole connecting the clubhead to the grip. The shaft has a tip-end (lower end) that is attached to the clubhead and has a butt-end (upper end) onto which the grip is attached. During use of the club for play, the golfer holds onto the grip while executing a “swing” aimed at striking and propelling the ball forward.
Conventional shafts generally are tubular with a circular cross section that progressively decreases (tapers down or steps down) to provide a correspondingly progressive decrease in shaft stiffness from the butt-end to the tip-end. Conventional shaft materials include any of several suitable metals (e.g., steel) or composite materials. Composite shafts tend to have a different, usually “softer,” feel than metal shafts, but tend to have lower mass than metal shafts. Composite materials usually are fibrous or filamentous materials reinforced with a cured synthetic resin. Most composite materials include some amount of carbon fiber (“graphite”) or other suitable fiber impregnated in the resin.
The United States Golf Association (U.S.G.A.) currently has rules that limit certain aspects of golf-club shafts. For example, the shaft must be straight from the top of the grip to a point not more than five inches above the sole of the clubhead. Also, at any point along its length, the shaft must exhibit a deflection that is the same regardless of how the shaft is rotated about its longitudinal axis and must twist the same amount in both rotational directions (clockwise and counterclockwise around the longitudinal axis). These rules impose certain limitations on the configuration and permissible behavior of the shaft during play. For example, conforming shafts exhibit both dimensional and material symmetry (about their longitudinal axes) as well as symmetrical flexing behavior during actual play.
All golf-club shafts flex over their length during a swing. Shafts have a characteristic, termed “flexional rigidity,” which at a location on the shaft is the product of Young's modulus (E) for the shaft material and second moment of area (I) of the section at that location. Normally, for shafts having a substantially cylindrical-tube shape, I=π(D4−d4)/64, where D is the external diameter and d is the internal diameter of the shaft at the location. In many types of clubs the flexional rigidity decreases substantially in a linear manner from the butt-end to the tip end of the shaft. Conventional shafts of metal-wood types of clubs have a flexional rigidity EI at the butt-end generally in the range of 50 to 180 N·m2, and at the tip-end about 10 to 30 N·m2.
In the quest to make golf more accessible and enjoyable to more players, attention has been given to altering the conventional configuration of golf clubs in the hope of making clubs more tailored to particular players and/or generally improving the performance of the clubs. For example, substantial effort has been directed to altering the distribution of discretionary mass in clubheads, altering the volume of clubheads, changing the material(s) of which the clubheads are made, and so forth.
One factor having a relationship to shaft flexibility, particularly of the shafts of metal-wood type clubs, is “dynamic loft.” Each of the different golf clubs in a set of clubs has a specified “loft,” which is the angle of the strike plate from a vertical plane when the clubhead is stationary at the address position relative to the ball. Thus, the club's stated loft provides the golfer with approximate information on the expected launch angle of a ball hit by the club. However, a club's loft number is simply a physical angle; other factors (in addition to the club's specified loft) contribute to the loft actually exhibited by the club during use, i.e., the dynamic loft or actual launch angle of the club. Dynamic loft affects, in turn, the flight of the ball, including flight distance.
Another factor related to shaft flexibility is “droop,” which is the deflection of the shaft, in the toe-down direction, perpendicular to the swing plane at the moment of impact with the ball.
A key determinant of dynamic loft, droop, and certain other behaviors exhibited by a clubhead during play is the flexibility of the shaft. As the golfer executes a swing, the clubhead accelerates from zero to high velocity (e.g., up to 80-100 mph) in a fraction of a second while sweeping radially in a substantially full-circular path as a result of force applied by the golfer to the grip. Hence, the shaft naturally flexes during the swing. The flexure results in changes in the orientation of the clubhead relative to the shaft (and to the ball) at the moment of impact, compared to a clubhead that is stationary adjacent the ball. The multi-variate effects of shaft flexibility can be complex and difficult to predict and model.
Some past development effort, aimed at improving club performance, has been directed to the shafts of golf clubs. For example, various attempts have been made to alter the EI profile in one or more selected regions of the shaft, i.e., to depart significantly from the normally substantially uniform rate of increase in flexibility down the length of the shaft. In this regard, U.S. Pat. No. 4,319,750 discusses composite shafts having increased flexibility in the butt-end region of the shaft, in the region of the grip. U.S. Pat. No. 5,439,219 discusses shafts having increased flexibility in a zone situated just downstream of the butt-end region, namely just below the grip. U.S. Pat. No. 7,070,512 discusses, with respect to certain wood-type clubs, shafts having increased flexibility (decreased stiffness) in a zone located upstream of the tip-end. The subject clubs have stated lofts ranging from 14 to 18 degrees. The low-stiffness zones have EI values of 5-10 N·m2, and the shafts preferably include a small region of increased stiffness between the increased-flexibility zone and the clubhead. In other words, the rigidity of the shafts increases from the low-stiffness zone both toward the clubhead and toward the butt-end.
In view of the large influence of subjective criteria, categorically termed “feel,” in the use of golf equipment, the various shaft-flexibility alterations noted in these references may be acceptable to certain golfers for certain golfing situations. But, for other golfers and/or other golfing situations, the alterations are not acceptable or effective.
Other factors affecting dynamic loft include prevailing weather conditions (wind, moisture, temperature), peculiarities of the golfer's swing, the spin imparted to the ball as struck by the club, and the particular golf course being played upon. (Spin is also affected, in turn, by the dynamic loft.) Since these factors are subject to change, it would be advantageous if the shaft flexibility of a particular club could be configured in a way that would yield a significant change in launch angle and ball spin. Pending a change in the relevant U.S.G.A. rules, it would also be advantageous if the flexibility of a region of the shaft of a particular club could be altered by the player in a way providing a degree of control over the effects of these factors. Certain advantages also could be realized if a club were provided having a shaft of which the local flexibility could be selectively manipulated for different golfer and/or to address situations arising during play.